Spectrum analyzers have an upper limit to how wide a frequency span can be acquired and processed at one time. Such upper limit is caused by speed and processing limitations of the hardware, and is particularly applicable to digital spectrum analyzers, where a band of signal is acquired and transformed into a frequency spectrum for that band. When a frequency span that is wider than can be accommodated in one acquisition is desired, multiple acquisitions with different acquisition and processing settings must be processed and the resulting two or more partial spectrums are subsequently “stitched” together.
FIG. 1 illustrates a conventional approach for multiple acquisitions and stitching of uniform frequency bands to provide a full spectrum. An RF input 105 receives a signal, which is attenuated or amplified using attenuator/amplifier 110. The signal is directed to different paths using switch 115. The paths correspond to different frequency bands 1 through N. Filters 120 are associated with the paths. All bands are of the same width, or at least substantially the same width. In other words, the frequency ranges for each path are uniform in their width with respect to each other. This is shown in the bands 170 that are output as spectrum 165. From 2 to N bands are stitched to create the wide span spectrum 165. Although the bands 170 are shown abutting one another, some overlap is necessary. In the conventional art, the overlap is minimal, on the order of 1%.
Bands 2 through N are sent to the mixer 130 using switch 117. The mixer 130 mixes the bands 2 through N with a local oscillator signal 125. Low pass filter 135 removes local oscillator feed-through and image spectrums that result from the mixing process. The analog-to-digital converter (ADC) 145 receives bands 1 through N from switch 140, and digitizes them. After the ADC, the signal may be further conditioned by a digital down converter (DDC) section 160, which may adjust the center frequency and reduce the sample rate. The DDC section 160 also includes a digital mixer 155 to mix the bands with a digital local oscillator signal 150 for each band, as well as other components for decimating or down sampling each individual band.
The bands are then transformed using transform 158, which can be a Fourier transform, among other suitable transform operations. The different spectrum bands are stitched together and the switch 162 outputs the bands 170 as spectrum 165. The switch 162 can be a virtual switch, implemented in software. Multiple stitches require multiple acquisitions. Each additional stitch requires a corresponding additional acquisition.
For performance and quality reasons, it would be desirable to minimize the number of stitches. With very wide overlapping bands, the bands can be mixed and matched to minimize the number of acquisitions, and therefore, stitching can be made less frequent. It would also be desirable to have non-uniform frequency ranges associated with each band, so that a user specified span is more likely to fall within one or more of the non-uniformly configured bands. It would be desirable to optimize downstream processing of each band by processing each band as if the full band was present.